Obesity is the strongest acquired risk factor for type 2 diabetes (T2DM), however the precise mechanisms linking these conditions are not known. The central hypothesis of this pilot project is that a failure to differentiate new subcutaneous adipocytes in response to over nutrition leads to the development of hypertrophic adipocytes that manifest alterations in the expression and secretion of adipokines. These changes, including increases in pro-inflammatory cytokines and decreases in adiponectin, provide feedback through both paracrine and endocrine axes to limit further fat accretion, at the expense of worsening metabolic dysfunction. Four specific aims will be tested in obese insulin resistant subjects (OIR, N = 15) and obese insulin sensitive subjects (OIS, N = 15) to determine: 1) whether preadipocyte proliferation or differentiation is impaired in vivo in OIR vs. OIS, 2) whether preadipocyte proliferation or differentiation is impaired in vitro in OIR vs. OIS, 3) whether impairments in preadipocyte proliferation are associated with hypertrophic adipocytes and a pro-inflammatory phenotype and 4) the role of the endocannabinoid system in regulating preadipocyte proliferation and differentiation. Subjects will undergo measurement of body composition (DEXA), oral glucose tolerance, insulin sensitivity (glucose clamp) and percutaneous needle biopsies of subcutaneous fat. Adipogenesis will be measured in vivo by determining turnover rates of adipocytes and stromal-vascular cells using 2H2O to label DNA. Proliferation and differentiation of preadipocytes in vitro will be measured in primary cultures derived from each subject. Adipocyte size, expression of pro-inflammatory cytokines and the expression of genes involved in the endocannabinoid pathway will be compared in OIR and OIS subjects and related to measures of proliferation and differentiation. Understanding the mechanisms whereby obesity leads to T2DM may improve risk stratification and may suggest novel approaches to prevent and treat T2DM and its complications. Project Narrative: Obesity is a potent risk factor for developing type 2 diabetes. This study tests whether differences in the growth rates of fat cells are related to the risk for diabetes in obese individuals. Results from this study could suggest new prevention / treatment strategies.